NavList:

Dear Lars,

Thanks for your quite interesting example.

I decided to run it with modern computation tools and making the best use of all explicit and implicit data.

(1) - Checks on the 1908 AA/NAL data

First, on Aug 14th, 1908 at 11h31m02,0s GMT  , i.e. at 23h31m02,0 UT1  /  23h31m10.5s TT , I am computing :

GAST : 21h02m56,7s (vs. 21h02m57s)

Altair : RA  / Dec  = 19h46m19,8s/N08°37'40.2" (vs. --/N08°37'39.8")

Moon : GHA/DEC/HP/SD  00h12m00.3s/-04°17'03"/60.39'/16.454' (vs. 00h12m00s/-04°17.1'/60.38'/16.5')

As a first conclusion, the 1908 AA/NAL data are excellent for CelNav purposes.

(2) - Successive approximations with Ship speed made good assumed at 0 kt

(2.1) - Assuming both observations made at 23h31m02,0s UT

Then we get Altair True Altitude at 67°53.3' (vs. 67°53.3') and Moon True Altitude at 21°19.2' (vs. 21°19.3') and an approximate fix at N30°23.0'/W014°55.5' with Altair Azimut at 191.2° and Moon Azimut at 108.8°.

(2.2) - "Recovering" Altair Azimut at 180°

We need to use an earlier time for the Azimut of Altair to be at 180°

From Position (2.1) by trial and error, we determine that Altair was azimut was due south - i.e. at 180.0° - at 23h14m10,0s UT

Hence with Altair : UT 23h14m10s UT/ Hs 67°57.5' and Moon : UT 23h31m02s / Hs 20°12.5' we get the following Fix : 23h31m02s UT : N30°44.3'/W014°47.0'  from which :

- The Moon Azimut was at 109.0° . And:

- The Altair Azimut was at 180.4° . We are happy with that figure and do not iterate further.

We can observe that this last determination si very close from Lars's published position at N30°44.0'/W014°46.7'

(3) - Reasonable estimate of Ship's average course and bottom speed

From Hamburg (N53.5°/E101°) to N49°/W006° : there are 663 NM on a Great Circle.

From N49°/W006° to current approximate position at N31°/W015°, there are1154 NM on a Great Circle.

Given the prevailing winds, let's assume that for a total trip length of 16 days, 8 days were required to reach N49°/W006°.

It leaves us with another 8 days to proceed from N49°/W006° to N31°/W015°.

We can reasonably assume that Prompt's average Speed and Course made good are 6 kts / 200° which certainly is not unrealistic.

Let's then compute a fix with the following data :

Altair : UT 23h14m10s UT/ Hs 67°57.5'   . And:

Moon : UT 23h31m02s / Hs 20°12.5' ; And :

Speed made good : 6 kts , and Course made good / 200°

We then end up with the following Fix at 23h31m02s UT : N30°42.7'/W014°47.7' 

which is probably our best guess for the finnish barque Prompt's actual position.

Captain Hugo Lundqvist position at N30°44'N / W14°46.7' is excellent in comparison of this "reprocessed" result.

It also show us that the tabular methods used then yield excellent results in this specific example.

Thanks again to you Lars for this quite instructive example.

Kermit